1. Field of the Invention
The present invention relates to an improvement in pyrotechnic gas generators or inflators of the type that utilize the combustion of solid fuel gas generant pellets for the generation of gas for rapidly inflating vehicle passive restraint inflatable crash protection bags.
2. Description of the Prior Art
Many forms of pyrotechnic gas generators or inflators are known in the prior art. These, in general, include an ignition charge, a gas generant charge of a composite solid propellant comprising an organic binder, an inorganic oxidizing agent and additives such as a metal fuel, and suitable cooling and filtering provisions.
Pyrotechnic gas generators with concentric chambers including a central ignition chamber, a surrounding combustion chamber containing the gas generant charge, and an outer cooling and filtering chamber are known in the prior art. Forms of such gas generators are disclosed in U.S. Pat. No. 3,985,076 granted to Fred E. Schneiter et al. and in U.S. Pat. No. 4,296,084 granted to Gary V. Adams et al., both of which patents are assigned to the assignee of the present invention.
The gas generant composition is commonly incorporated in such prior art pyrotechnic gas generators in the form of small pellets having the general shape and size of aspirin tablets. Since these pellets are of uniform size with all of them having substantially the same surface area, the total burning surface area of the gas generant charge can be known or readily determined. Hence, when the gas generant charge is ignited, the rate at which gas is generated can be predicted with considerable accuracy. With age, and continued vibration with mutual pellet abrasion, however, there is a tendency for a fine powder of the gas generant composition to be produced. This can result in a much larger composition burning surface area than intended, and as a result, a possible overpressurization of the inflator.
An arrangement for solving this problem is disclosed in copending application for U.S. patent bearing Ser. No. 595,803, filed on Apr. 2, 1984 by Gary V. Adams et al., now U.S. Pat. No. 4,561,675, issued on Dec. 31, 1985, and assigned to the assignee of the present invention. In that application, there is disclosed a pyrotechnic gas generator having an annular chamber for the gas generant pellets. A press fitted annular disc or plate having a soft or sponge-like pad made of cerafiber on the side adjacent the pellets is pressed against the latter for holding them in place. As the pellets continue to settle against one another, however, so that the entire gas generant charge occupies less space, the annular plate can no longer apply pressure to the pellets. As a result, the plate is then ineffective to prevent mutually relative movement of the pellets.
It has also been proposed in the prior art to use a layer of foam for this purpose, but plastic or rubber foam tends to deteriorate with age, so that it is no longer resilient. Additionally, when the gas generant is ignited, such foams tend to produce toxic gases and/or disagreeable odors.
Long duration stability of pyrotechic gas generators is needed because of the expected longevity of use, which may be ten (10) years or more, of the vehicle in which the gas generator is installed. Thus, there is a need and a demand for an improvement in such generators for overcoming or at least minimizing the tendency toward degradation of the gas generant pellets with age and continued vibration.